1. Field of the Invention
The present invention relates to an improved apparatus for injection molding hollow plastic articles and an improved method of molding hollow plastic articles.
2. Description of the Prior Art
The manufacturing process of injection molding has been used for many years to manufacture in bulk a wide variety of different types of plastic articles. Many of these articles are hollow structures. Articles may be made hollow in order to fit onto some other structure. Also, it is often much more economical to manufacture articles in a hollow rather than a solid form, both to reduce the weight of the plastic part and also to reduce the material expense involved in the manufacturing process.
For many years hollow plastic parts have been injection molded utilizing a mold comprised of two or more molding blocks having depressions therein which, when placed together, form a mold cavity. The molding blocks are separable along a parting interface. Prior to the introduction of molten plastic, a core structure is placed in between the molding blocks. Depressions in the molding blocks form one or more seats to hold the core in a proper, predetermined position within the mold cavity. The molding blocks are then closed upon the core to at least partially encapsulate it within the mold cavity. Molten plastic is then forced under pressure through a passageway formed by mating, concave channels in mating surfaces of the molding blocks to enter the mold cavity through a duct called a molding runner gate. The molten plastic is then forced into the mold cavity surrounding the core and fills the entire volume of the mold cavity except that portion of the volume of the mold cavity occupied by the core. The mold is then cooled so that the plastic solidifies to form a molded plastic structure. The core is then removed from within the molded plastic structure.
One problem that frequently arises when fabricating injection molded plastic parts in this manner is that a noticeable mark is formed on the area of the exterior surface of the plastic article that is formed at the runner gate. This mark may take the form of a protrusion, an indentation, or some other surface defect. In any event, the appearance of such a surface blemish is often unacceptable to the customer for whom the part is manufactured.
To remedy this defect the fabricators of plastic articles have sometimes attempted to introduce the molten plastic into the mold cavity through the mold core. To do this the mold core must be hollow in order for the molten plastic to flow through it. In such a system the molten plastic flows outwardly from the hollow center of the core through one or more ejection gates and into the mold cavity from the outer surface of the core, which forms the interior surface of the hollow plastic article. Since the interior surface of the plastic article is normally not visible, the appearance of surface blemishes on the interior surface of the molded article at the location of the ejection gates is normally not a matter of concern.
However, certain difficulties and disadvantages exist with the use of conventional molding systems of this type. Since the molten plastic is conducted through a passageway in the core, a certain amount of plastic is left within the passageway in the core once the molded plastic article has been formed. Plastic left in the core passageway is termed a xe2x80x9crunnerxe2x80x9d and must the removed before the core can be used again. One conventional technique to accomplish this is to maintain the core in a heated condition even after the hollow plastic article has been formed so that the runner remains in a melted condition and can be poured in molten form from the core. Even in molding a plastic article in a single shot an additional amount of thermal energy is required, thus adding to the expense of the manufacturing process. Also, a step of inverting the core to pour the molten plastic from it is often required. The core is unavailable for service in manufacturing a subsequent hollow plastic part during the time that the melted plastic is being poured from it. As a consequence, the throughput of fabricated molded plastic articles is reduced. These problems are quite significant in the competitive field of injection molding.
This conventional technique of injection molding hollow plastic articles is particularly unsatisfactory when the articles to be fabricated are molded in several stages using plastics having different characteristics, for example different colors. In such a situation it is necessary not only to pour the molten runner from the core in order to remove it from the core, but it is also necessary to xe2x80x9cpurgexe2x80x9d the core as well. That is, a certain amount of the next color of plastic to be used in the next stage of fabrication must be introduced into the runner passageway of the core and then poured out of the core while still in molten form in order to collect and carry with it residual amounts of the plastic used in the prior stage of fabrication. The plastic which is employed in the purging operation is discolored and is unsuitable for reuse. It must therefore be discarded as waste. This adds significantly to the cost of manufacturing each multicolored plastic article.
Furthermore, even with purging, not all of the prior plastic in the runner passageway comes out of the core. As a consequence, discoloration in the next subsequent shot of plastic is quite common. Moreover, since the core remains heated with the layer of plastic formed during the earlier stages of manufacture still on it, a loss of definition is likely to occur because the heated core maintains the preliminary portion of the article formed in a soft condition on the exterior surface of the core. This loss of proper definition in the shape of the portion of the article manufactured in the earlier stages of fabrication occurs because it is not possible to maintain the interior of the core in a heated condition without softening the portion of the plastic products already formed on the outer surface of the core. This produces a product of inferior quality. Moreover, because the core never really cools, the several layers of plastic molded onto the core tend to remain at too high a temperature. This causes them to mix at their interfaces, thus creating a further loss of definition at the interfaces between the different colors of plastic.
The present invention involves a system of injection plastic molding in which the molten plastic is injected into the mold radially outwardly from within a hollow, metal core. The core has opposing ends, at least one of which is an open end. The core has a longitudinal axis that extends between the opposing ends. The core has an outer surface, and defines within its interior, a core runner cavity that has a uniform, longitudinal cross section throughout relative to the longitudinal axis of the core. The core runner cavity extends through the open end of the core. A removable core end closure is located at the open end of the core and defines a core runner extension cavity within its structure. The core runner extension cavity of the end closure is in communication with the core runner cavity and is aligned on the longitudinal axis of the core.
Once the molten plastic has been injected into the mold cavity through the hollow core located therewithin, the mold halves are parted so that the core can be removed. Moreover, and unlike prior systems, the core does not need to be maintained in a heated state so that the plastic from within the core can be drained therefrom. Rather, the mold is allowed to cool. Even though the elongated runner within the hollow core cools, it can still be removed from the core even in a solidified state by merely uncapping the core, gripping the portion of the cold runner that has formed in the core runner extension cavity, and pulling the cold, solidified runner longitudinally out of the hollow core. The plastic runner is resilient enough so that it will pull free of the core. Indeed, the application of longitudinal tensile stress to the structure of the cold runner by pulling on the end of the cold runner that protrudes from the open end of the core has the effect of stretching the runner, thereby reducing its cross-sectional area. This facilitates its separation from the interior walls of the core runner cavity.
The present invention has significant advantages over the conventional hot runner extrusion molding systems. By allowing the core to cool at each stage of multiple stages of molding, the definition of each portion of the molded plastic structure is preserved so that there are clear demarcations between the portions of the article molded at different stages of the fabrication process. There is no mixing of colors at the interfaces between plastics of different colors as occurs in hot runner molding systems.
A very significant advantage of the present invention is that it avoids the step of purging entirely. Because the runner is removed from the core in cold, solidified form, no residue of plastic is left within the runner passageway or the ejection gates to contaminate the next shot of plastic of a different color injected into the core. As a consequence, mixing of colors from sequential shots of injection molded plastic is avoided entirely.
In one broad aspect the present invention may be considered to be a molding apparatus for injection molding plastic articles. The apparatus of the invention is comprised of an outer mold, a hollow mold core, and a removable core end closure. The outer mold is comprised of a plurality of molding blocks which are separable along a parting interface and which, when positioned together, define an enclosed article mold cavity therewithin. The hollow mold core has opposing ends, at least one of which is an open end. The mold core has a longitudinal axis that extends between the opposing ends. The mold core has an outer surface and also defines a core runner cavity therewithin. The core runner cavity has a uniform longitudinal cross section throughout relative to the longitudinal axis of the core. The core runner cavity extends through the open mold core end. At least one core ejection gate is defined within the core leading from the core runner cavity to the outer surface of the core. The removable core end closure is positionable at the open end of the core and defines a core runner extension cavity therewithin. The core runner extension cavity is in communication with the core runner cavity and is aligned on the longitudinal axis of the core.
The mold core is held in a fixed, predetermined position relative to the article mold cavity by a seat for the core defined by the molding blocks of the outer mold. A path of injected molten plastic flow is established into the core runner cavity, through the core ejection gates, and into the article mold cavity from inside the mold core when the core is within the outer mold and the mold blocks are positioned together.
The invention has particular applicability to the production of molded plastic articles formed in a sequence of stages utilizing plastics having at least one different characteristic at each stage. For example, the invention has particular applicability to forming hollow plastic articles formed from shots of plastic which are different in color, density, hardness, resiliency, permeability, or some other physical or chemical characteristic.
For molding plastic articles utilizing plastics having different characteristics which are injected in sequence during different stages of fabrication, the invention may be considered to be a molding apparatus for injection molding hollow plastic articles comprising a plurality of outer molds, a hollow mold core, and a removable core end closure. The outer molds are each comprised of a plurality of molding blocks which are separable from each other along a parting interface and which, when positioned together, define an enclosed article mold cavity. The mold cavities of the plurality of outer molds are each of a different shape and a different volume.
The hollow mold core has opposing ends, at least one of which is an open end. The mold core defines a longitudinal axis extending between the opposing ends. The mold core has an outer surface and defines a core runner cavity therewithin that has a uniform cross section throughout along the longitudinal axis. The cold runner cavity extends through the open mold core end. A plurality of core ejection gates are defined within the core leading from the core runner cavity through the wall of the core to the outer surface of the core.
The removable core end closure is positionable at the open end of the core. The core end closure defines a core runner extension cavity therewithin that is in communication with and longitudinally aligned with the core runner cavity. When the core is positioned within at least one of the outer molds, the molding blocks thereof block flow from at least one of the plurality of core ejection gates. More specifically, the molding blocks for all of the outer molds, except the outer mold for the last stage of injection molding, are configured to block at least one of the core ejection gates. As each stage of the plastic article is molded, the overlying portions of the article already formed will block flow through the core ejection gates used to form those portions. During the last stage of molding, the portions of the article already formed block all of the core ejection gates with the exception of those needed to form the final portion of the article.
Contamination and discoloration of subsequent stages of molding is thereby avoided. A molten plastic having a different physical characteristic, such as a different color, is employed at each stage of molding. Projections from the interior mold cavity walls of the mold blocks of each stage, except the final stage, block at least one core ejection gate at each molding stage except the final molding stage.
In another broad aspect the invention may be considered to be a method of molding a hollow plastic article utilizing at least one outer mold, a hollow mold core, and a core end closure. Each outer mold is comprised of a plurality of molding blocks which separate along a parting interface and which, when positioned together, define an enclosed outer mold cavity therewithin. The hollow mold core has opposing ends, at least one of which is an open end. The hollow mold core defines a longitudinal axis extending between the opposing ends. The mold core has an outer surface and defines a core runner cavity therewithin. The core runner cavity has a uniform cross section throughout relative to the longitudinal axis of the core. The core runner cavity extends through the open mold core end. At least one core ejection gate is defined within the core leading from the core runner cavity to the outer surface of the core. The core end closure is positioned relative to the core to block the open end of the core. The core end closure defines a core runner extension cavity therewithin that is in communication with the core runner cavity and is aligned on the longitudinal axis of the core.
The steps of the method of the invention comprise: withdrawing the core blocks from each other; placing the mold core between the molding blocks; positioning the molding blocks together, thereby clamping the mold core therebetween at least partially within the outer mold cavity; closing the open end of the mold core with the core end closure; injecting molten plastic into the outer mold cavity through the core runner cavity and the core ejection gate; cooling the outer mold and the mold core, thereby solidifying the molten plastic injected into the outer mold cavity into a molded plastic structure mounted on the mold core and thereby solidifying the molten plastic injected into the core runner cavity and the core runner extension into a solidified core runner occupying both the core runner cavity and the core runner extension cavity; removing the core end closure from the open end of the mold core, thereby exposing that portion of the core runner solidified in the core runner extension cavity; gripping the core runner by the exposed portion thereof and drawing the solidified core runner along the longitudinal axis completely out of the core runner cavity; drawing the molding blocks apart; and thereafter removing the molded plastic structure from the mold core.
The method of the invention has very considerable advantages as contrasted with conventional hot runner injection molding systems when implemented to form plastic articles with different portions formed of different plastics having at least one characteristic different from each other. For example, considerable advantages are to be gained by using the invention to form plastic articles with portions of different colors.
To fabricate articles having portions formed of different plastics at least first and second stage outer molds are utilized in the performance of the steps previously described. The molding blocks of the first stage outer mold have interior walls that define a first stage mold cavity and include port-blocking projections that extend into the first stage mold cavity. The mold blocks of the second stage outer mold have interior walls that define a second stage mold cavity which is larger in volume than the first stage mold cavity. The second stage mold cavity totally encompasses the first stage mold cavity. The mold core which is utilized has a plurality of core ejection gates defined within the core leading from the core runner cavity to the outer surface of core.
To mold a plastic article utilizing first and second outer mold stages, all of the steps of the invention previously described, except the final step of removing the molded plastic structure from mold core, are first performed utilizing the first stage outer mold. During the step of injection in the first stage at least one of the core injection gates is blocked by projections formed on the interior walls of the molding blocks of the first stage outer mold. These projections are brought into position to block one or more of the core ejection gates as the core blocks of the first stage outer mold are brought together to clamp the core mold therebetween.
After the first stage of molding both the molded plastic structure formed on the outer surface of the core and the core runner are cooled so that they solidify. At this point a portion of the plastic article is formed on the outer surface of the core during the first stage of molding. The end closure is then removed from the open end of the core. The portion of the core runner formed within the core runner extension cavity during the first stage of molding is thereupon exposed. This portion is gripped and pulled longitudinally thereby drawing the entire core runner out of the core runner cavity. The thermoplastic utilized is soft enough so that, as tensile stress is exerted, the core runner is stretched and the cross-sectional area of the core runner is reduced. This facilitates separation of the core runner from interior walls defining the core runner cavity. Also, the thermoplastic utilized is soft enough so that the short, radially projecting branches from the main body of the core runner produced by the thermoplastic remaining in the core ejection gates will pull free from the core ejection gates and will be withdrawn along with the main body of the core runner. The molding blocks of the first stage outer mold are then drawn apart.
All of the steps of the method of the invention are then performed utilizing the second stage outer mold and utilizing a second plastic in the step of injection that has at least one characteristic different from the molten plastic employed in the injection step utilizing the first stage outer mold. If the method of the invention is performed utilizing only two stages of molding, the molded plastic structure is removed from the mold core following the second stage of molding. However, the invention may be performed with three, four, or even a greater number of stages of molding. In any event, the plastic structure is built up on the core at each molding stage and remains on the core until after the final stage of molding has been performed.
The invention may be described with greater clarity and particularity by reference to the accompanying drawings.